A. Field of the Invention
The present invention relates to a protective film of a magnetic recording medium.
B. Description of the Related Art
Existing magnetic recording media have a protective film composed of a diamond-like carbon (DLC) film deposited on a magnetic recording layer by means of a plasma CVD method in order to protect the magnetic recording layer against damage resulting from contact with and sliding of a magnetic head and corrosion of the magnetic recording layer. The DLC film exhibits such a good surface smoothness and large hardness as to be suited for a coating film. The DLC films are formed by means of a sputtering method and a plasma CVD method.
Magnetic recording media are now changing from the longitudinal recording system to the perpendicular recording system, the latter being capable of higher recording density than the former. For enhanced recording density of a magnetic recording medium, the perpendicular magnetic recording system, like the longitudinal recording system, needs to have a reduced distance (a magnetic spacing) between the magnetic recording layer and the magnetic head. A thickness of a protective film formed on the magnetic recording layer accounts for a portion of the magnetic spacing. It is therefore necessary to reduce the thickness of the protective film to be as thin as possible to enhance the recording density while ensuring the function of a protective film.
To ensure the protective function of such a thinned film, the protective film must be as dense as possible. A DLC film formed by a plasma CVD method is denser than a DLC film formed by a sputtering method. A filtered cathodic arc (FCA) method is one of the methods capable of forming a further dense carbon film.
The FCA method uses a cathode target of polycrystalline graphite, which is vaporized and ionized by arc discharge. A magnetic filter sorts out only carbon ions, which are then transported to a target substrate for film deposition. Since the carbon film is formed on the target substrate only with carbon ions, the FCA method can form a tetrahedral amorphous carbon (ta-C) film which does not contain hydrogen. The resulting ta-C film is denser than a DLC film.
Using this ta-C film in a magnetic recording medium, a protective film is expected to be accomplished that is thinner than a protective film composed of a DLC film formed by a plasma CVD method. The ta-C film formed by the FCA method, however, has a problem in that it contains a large amount of aggregate particles.
In order to reduce the aggregate particles, Japanese Unexamined Patent Application Publication No. 2004-244667 discloses a means for capturing the particles which employs a shielding structure called a baffle in the above-mentioned magnetic filter.
To reduce further the aggregate particles formed in the FCA method, Japanese Unexamined Patent Application Publication No. 2004-256837 discloses a structure comprising a plasma forming part between a magnetic filter and a film deposition chamber, the plasma forming part transforming the carbon aggregate particles passing through the magnetic filter into carbon ions.
Japanese Unexamined Patent Application Publication No. 2002-334428 discloses a method for generating carbon ions by irradiating a solid-state carbon source composed of graphite with KrF excimer laser light in place of an arc discharge in the FCA method.
Moreover, the flying height of the magnetic head, which accounts for a part of the magnetic spacing as well as the protective film thickness, decreases each year in order to enhance the recording density. Thus, the reduction of aggregate particles in the protective film has been required to be more rigorous each year. Conventional technologies cannot fully meet this rigorous requirement for reduction of the aggregate particles.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.